For a filter that is used in an imaging element in an imaging apparatus, such as a digital camera, for example, a Bayer arrangement and the like in which R, G, and B colors are arranged is used in most cases, but a filter has been proposed that has an RGBW arrangement including all-wavelength transmission type white (W) pixels, which includes all wavelength areas of the R, G, and B in addition to the R, G, and B colors.
However, problems with reduction processing of noise that is included in an image that is captured using the RGBW arrangement filter and that has the white (W) pixels are as follows. Color pixels of the R, G, B and the like are lower in sensitivity and are greater in an amount of noise than the white (W) pixels, and furthermore, the number of reference pixels that are usable to calculate a compensation pixel value of an attention pixel that is a noise reduction target, that is, the number of samples of the reference pixels which have the same color as a compensation target pixel is small. As a result, there is a problem in that even though the noise reduction (NR) processing that refers to the pixels which have the same color is executed, a sufficient noise reduction effect cannot be obtained.
Furthermore, if an object is to address a model, like a light shot noise, in which noise changes according to optical strength, this causes a problem in that smoothing strength varies widely due to noise staying on the pixel itself and a signal level is lowered.
Furthermore, because color-pixel sampling positions are scattered in defect compensation in a white (W) arrangement, it is difficult to distinguish between a defect and a texture, and there is a problem in that an effect of sufficient compensation cannot be obtained.
Moreover, as the related art relating to the processing that reduces noise in the image is disclosed, there are, for example, PTL 1 (Japanese Unexamined Patent Application Publication No. 2003-224861), PTL 2 (Japanese Unexamined Patent Application Publication No. 2011-76186, and the like.
PTL 1 (Japanese Unexamined Patent Application Publication No. 2003-224861) discloses a configuration in which noise is reduced by reducing a frequency component of a color difference signal (C) of each area according to a frequency component strength of a luminance signal (Y). However, because this technique performs reduction of noise in the C signal, based on the Y signal even at a point where there is no relationship between the luminance signal (Y) and the color difference signal (C), there is a concern in that the color difference signal will be lost at a point where a luminance change rarely occurs, such as a color texture.
In PTL 2 (Japanese Unexamined Patent Application Publication No. 2011-76186), there is disclosed a technique that performs determination of a texture direction using a W pixel, and based on the result of the determination, performs the defect compensation. PTL 2 discloses the technique of performing defect compensation on the W pixel, but not a method of performing compensation on color pixels other than W. Furthermore, there is a problem in that an arithmetic operation cost is increased because of a variety of direction determination processing subject to two-dimensional processing that refers to pixels in upward, downward, leftward and rightward directions on a two-dimensional plane.